Copy AN nodes to another file
The only way I know of it's importing (appending in blender terms) the entire node-tree :/
File->Append->select the blend file to copy from-> click NodeTree-> click the tree of interest.
Definitively not user friendly considering the more common case to import into your current tree, but you can invoke the subprograms inside the newly imported node-tree and use it in your main tree.
Another way would be to add the code to recreate to the templates dir and repackage-reinstall the addon (or just add them directly in your addon's  installation directory) for example:

    [...]
    sourceObjectNode = self.newNode("an_DataInputNode", x = 0, y = 0, label = "Source Object")
    sourceObjectNode.assignedType = "Object"

    timeInfoNode = self.newNode("an_TimeInfoNode", x = 0, y = -200)
    [...]


See here for the full explanation of how to take advantage of multiple node-tree